HUMINT is the oldest intelligence discipline and the one that is most often written about in the media. The CIA is the primary collector of HUMINT, but the Defense Department also has responsibilities filled by defense attachés at embassies around the world and by other agents working on behalf of theater commanders.

In overt collection, the collector meets openly with sources as a declared U.S. Government representative. Overt collection comprises many forms of information collection, including debriefings of persons who have travelled to countries of national interest, diplomatic reports from embassies on host-country officials' stated reactions to U.S. policy initiatives, and law enforcement reports on criminal activities, such as drug trafficking.

Clandestine collection is conducted in secret. A clandestine collector must locate a person with access to desired information, initiate and discreetly develop a relationship with that prospective source, and ultimately convince the source to divulge secrets. A source may or may not be told of his interlocutor's U.S. Government affiliation. After the source is recruited, contact is usually strictly controlled in an effort to elude discovery. The recruitment of a clandestine human source can take months or years, but the leak of a source's information may immediately eliminate access to that source.

Many observers have argued that inadequate HUMINT has been a systemic problem and contributed to the inability to gain prior knowledge of the 9/11 plots. In part, these criticisms reflect the changing nature of the international environment. During the Cold War, targets of U.S. HUMINT collection were foreign government officials and military leaders. Intelligence agency officials working under cover as diplomats could approach potential contacts at receptions or in the context of routine embassy business. Today, however, the need is to seek information from clandestine terrorist groups or narcotics traffickers who do not appear at embassy social gatherings. HUMINT from such sources can be especially important as there may be little evidence of activities or intentions that can be gathered from imagery, and their communications may be carefully limited.